A device such as a smartphone, tablet or other mobile terminal may be capable of supporting multiple radio access technologies for communicating over a wireless cellular network. For example the device may support both 2G and 3G technologies; or 2G, 3G and LTE (Long Term Evolution) technologies. A radio access technology (or “RAT”) is a manifestation in a device and/or network of a particular set of standardised specifications, comprising standardised protocols for communicating over the network. For example 2G, 3G and LTE technologies are defined in the various releases of the 3GPP (3rd Generation Partnership Project) standard. Each of the radio access technologies can be used for both voice over a dedicated voice channel and data over a general purpose packet-based data channel.
In a 3GPP environment, the radio access technology used by the device at any given time is subject to precise rules specified in the standard. Criteria have to be applied for inter-RAT selection, and these criteria are controlled by the network. A control element of the network such as a radio network controller (RNC) makes inter-RAT decisions on behalf of the device based on the criteria as evaluated at the network control element according to the strict rules specified in the standard (even if based on measurements submitted on by the device on the uplink). Commands which impose the inter-RAT decisions on the mobile device in question are then sent on a control channel on the downlink from the network.
Typically, the modem of the mobile terminal itself can also take the decision to change RAT but only when radio quality under the current RAT falls below a certain acceptable threshold level. I.e. the mobile can change to another RAT if the current RAT there is effectively no longer suitable coverage under the current RAT for the connection to work properly, but in all other circumstances such decisions are imposed by the network